Usually, when a mass of cement clinker is calcined in a SP (suspension preheater) kiln or a NSP (new suspension preheater) kiln, volatile components such as chlorine, alkali and sulfur which have been brought by cement raw materials and fuel, are circulated within a preheater system so that they are continuously concentrated.
However, it is well known that this kind of circulation will reach equilibrium in several hours, a state where the amount of volatile components being brought into the preheater system from the cement raw materials and fuel is equal to the amount of volatile components being taken out of the system by the cement clinker.
Therefore, if the amount of volatile components being brought into the system from the cement raw materials and fuel is large, the amount of volatile components contained in the cement clinker also becomes large. This influences adversely on the quality of the cement product.
Further, if the amount of volatile components in the system increases, compounds having low melting points will be formed, causing possibly frequent clogging of the preheater system and working adversely against a stable operation of the kiln.
Recently, especially as the utilization of industrial wastes has been promoted, it is increasingly needed to utilize raw materials having high chlorine content, and efficient removal of volatile components has been expected.
So, in order to reduce the amount of volatile components within a kiln preheater system, the so-called by-pass installation has been provided to the system (e.g. Laid-open Japanese Patent Publication H2-116649).
The by-pass installation includes a rising duct connected on the rotary kiln inlet for discharging kiln exhaust gas, a probe whose front end abuts on the inside of, or preferably is protrudingly provided inside of, the rising duct for extracting a part of the kiln exhaust gas, and a system connected at the rear end of the probe for extracting and discharging the kiln exhaust gas.
The above probe is formed into a double-tubed structure including an inner tube connected to the gas extracting and discharging system, and an outer tube for introducing the atmosphere into the vicinity of the front end of the inner tube protruding into the rising duct. In the by-pass installation, the atmosphere is introduced into the rising duct through an air flow path formed between the outer tube and the inner tube, while at the same time a part of the kiln exhaust gas is extracted out of the rising duct together with the introduced air.
Further, it is known that, if rapidly cooled in the probe to a temperature of 600-700.degree. C. or lower, volatile components such as chlorine contained in the kiln exhaust gas will be concentrated to form fine powder entrained in by-pass dust which is generated by the kiln by-pass process.
It is therefore suggested that a by-pass dust separating means be provided downstream of the probe for separating coarse powder dust having low volatile component concentration and fine powder dust having high volatile component concentration, and feeding the former back to the kiln.
On the other hand, there has been developed a kiln by-pass technique which is able to greatly reduce the amount of by-pass dust by discharging only fine powder dust from the kiln preheater system.
With the technique, it is indispensable to provide a means for concentrating volatile components into fine powder dust, that is, a means for rapidly cooling the kiln exhaust gas in the probe.
In a conventional probe having a double-tubed structure, cooling air is caused to flow through an air path formed between an outer tube and an inner tube so that it flows into a rising duct connected on the kiln inlet, to cool the kiln exhaust gas to be extracted.
However, since there is a considerable degree of the so-called blow-by of the cooling air, it is not available to mix the cooling air with the kiln exhaust gas quite efficiently at the front end portion of the probe. As a result, it is difficult to effect an instant quick-cooling of the kiln exhaust gas at the front end portion of the probe.
So, when a conventional probe is used, the amount of the cooling air will be increased so as to enable quick cooling. However, increase of the cooling air leads to increase of the cooling air flowing into a preheater system, meaning air increase in the preheater system, and heat loss and electric power loss will result.
It is an object of the present invention to provide an improved method and apparatus capable of effectively quick-cooling a by-pass kiln exhaust gas to be extracted without causing heat loss, electric power loss, etc.